Whenever a disruption such as a fiber cut occurs in a communications network having a plurality of nodes interconnected via a plurality of communications spans to have a plurality of communications circuits, impaired signals representative of the disruption quickly propagate in a wide area of the network beyond the point of failure. This propagation makes it difficult to identify the location of the fault. But to reroute the affected communication circuits around the failure, it is necessary to identify the point of failure. Frequently, such identification is performed by a central network management system by means of a sophisticated analysis of the alarm messages received from the affected nodes, coupled with the network topology information stored at the central system. However, such centralized approach for fault location is relatively slow and possible restoration of disrupted communication traffic to normal operation is often delayed.
With the increasing deployment of intelligent network elements, such as digital cross-connect systems, at nodes in today's communication networks, it has become possible to perform the fault identification described above, and subsequent restoration of disrupted traffic to normal operation, faster using distributed techniques rather than centralized methods. One such distributed method for locating a failed span is described in Pekarske U.S. Pat. No. 5,233,600. Pekarske teaches a method whereby, when a span in a network fails, the two nodes at each end of the failed span could identify, without assistance from a central system, that they are the custodial nodes of the failed span. The other nodes affected by the failure of the span also would identify that the spans to which they are directly connected are not the source of the failure. This identification is based on monitoring each circuit in a given span for either a directly observable signal impairment or an alarm indication signal. An idle signal is sent whenever a directly observable signal impairment or an alarm indication signal is detected. And the channel immediately upstream from the node is declared to have failed when either the directly observable signal impairment or alarm indication signal is received for longer than a predetermined period of time.
Being able to identify the failed span in the manner as described in Pekarske is sufficient to carry out a distributed link restoration procedure (which is limited to rerouting disrupted communications circuits), by the two custodial nodes of the failed span. However, a shortcoming of the Pekarske method is that the knowledge of which specific span has failed is confined to those two custodial nodes. In other words, the other nodes in the network affected by the failure would have no knowledge of the identity of the failed span. Yet in order to effect a more efficient restoration technique which may be referred to as a distributed path restoration technique for restoring each disrupted communications circuit independently of other disrupted circuits, other information such as the identity of the two adjoining nodes bracketing the failed segment of the communications circuit also needs to be made known to all nodes in the network. One of the reasons that the distributed path restoration technique is preferred over the link restoration method is that the former potentially can explore a much larger number of restoration options than the latter. Another reason is that the distributed path restoration technique requires potentially a smaller amount of spare capacity in the network than the distributed link restoration technique for providing the same level of network restoration capability.